Sloshing Impact Response in LNG Membrane Carriers - A response analysis of the hull structure supporting the membrane tanks

Abstract

The growing environmental awareness during recent years has led to a significant increase in the use of natural gas as a green energy supply. This, in turn, has increased the demand for transportation of liquefied natural gas (LNG) and therefore LNG carriers are growing in both size and numbers. This master’s thesis deals with a challenging design issue for LNG carriers with membrane type cargo tanks; the determination of structural response due to sloshing impact loads. These loads are characterized by very short durations and are thus likely to inflict a dynamic amplification in the response of the hull. This thesis covers a study consisting of FE analyses performed on a model representing parts of an LNG membrane tank. The main objective was to find and quantify the dynamic amplification factor (DAF) for the structural response towards sloshing impact pressures. The influence of variations in the load characteristics such as load duration, extent of the loaded area, load location as well as the influence of the insulation system was evaluated. The analyses were performed using an explicit calculation scheme in the commercial finite element software ABAQUS. The study shows that the response in the studied region of the hull structure experiences significant levels of dynamic amplification during impact loads with specific durations. This has been found especially pronounced for certain load locations due to their excitation of structural members in the inner deck structure. The outcome of the response sensitivity analysis also shows that the insulation system (MARK III type) used for containing the cargo has a large effect on the dynamic behaviour of the hull structure. It has been found to alter the magnitude of the stress and deflection response for key structural members. It also changes the load time durations for which the maximum dynamic amplification occurs and increases the magnitude of the corresponding response DAF. It has been found that dynamic response, such as torsional deflection modes of the stiffeners and amplification effects from the insulation system, gives DAF values of up to 2. The effects have been found to be present for temporal load characteristics commonly occurring in sloshing model tests and full-scale measurements and are therefore likely to occur for a vessel in operation.